The expandable thermoplastic resin beads can be manufactured according to the process as shown below. For example, they are manufactured by impregnating polystyrene resin beads in the form of aqueous suspension, with a volatile aliphatic hydrocarbon enabling to slightly swell them such as n-pentane or the like, or by impregnating polystyrene resin beads in the form of aqueous suspension containing a small amount of such a solvent as toluene, cyclohexane or the like having solubility for the polystyrene resin beads, with such an blowing agent as butane, propane or the like the gaseous form at ordinary temperature.
The expandable thermoplastic resin beads thus obtained are previously expanded to give pre-expanded beads. Then, these pre-expanded beads are charged in a metal mold of a molding machine, and heated over the softening point of the pre-expanded beads by introducing steam therein. Thus, each pre-expanded bead is fused into one to obtain a molded article as per the metal mold.
When the beads in the resultant molded article are not completely fused into one, however, there is a possibility that, for example, if the article is used as a vessel, contents permeate into an outside wall of the vessel or into the molded article, depending on the kind of the contents. In order to prevent this permeation, there has already been proposed a method of lowering the surface energy of the bead, that is to say, a method of enlarging the contact angle of the surface of a molded article to the contents to be permeated.
For example, Japanese Examined Patent Publication No. Hei 4(1992)-53890 discloses a proposal of surface coating with a random, block or graft copolymer of a fluorine-containing vinyl-type monomer and a hydrophilic vinyl-type monomer, Japanese Unexamined Patent Application No. Hei 1(1989)-210435 discloses a proposal of surface coating with ammonium phosphate of perfluoroalkylsulfonamide, and Japanese Unexamined Patent Application No. Hei 3(1991)-190941 discloses a proposal of surface coating with a random copolymer of a fluorine-containing vinyl monomer containing a hydrophilic component (ethylene oxide or propylene oxide group) with a lipophilic monomer All the surface modifying agents described in these patent publications are fluorine-series surface modifying agents which contain a hydrophilic component or a hydrophilic group.
However, even if the fluorine-series surface modifying agents containing a hydrophilic component or a hydrophilic group were used, satisfactory results were not always obtained in regard to performance enabling to prevent the permeation of fat and oil components and aqueous surfactant solutions; and stickiness of expandable thermoplastic resin beads which occurs during transportation of the beads after the pre-expanding. At first, the former will be explained below.
The vessels manufactured by molding expandable thermoplastic resin beads have a wide range of utilities because of the economy and heat-retaining property. Therefore, they are used, for example, as vessels for containing oily foods such as instant noodles, fried chicken and the like. In this case, the permeation of oil and fat components may become a problem, because the contents contained in the vessel during preserving in stock in the marketplace may be affected under the influence of temperature and moisture so much that oil and fat components or soup may effuse out to the outside wall. Furthermore, if an aqueous surfactant solution having a low surface tension is contained herein, the content will have permeated into the container so rapidly that it will be difficult to use the vessel. The above-mentioned proposals were made to cope with these problems but they were not satisfactory enough in regard to shielding property.
Secondly, the latter will be explained. A hydrophilic component or hydrophilic group is introduced in the surface modifying agent to prevent oil permeation, but it still accompanies a problem that the hydrophilic component absorbs water, which will cause the surface of the bead to be sticky. When such beads were pre-expanded, the beads become sticky after the pre-expanding so that the flowability of the beads tends to worsen. Therefore, there was a problem that it took a long time transport the beads on a net to remove fused beads in a bead-transferring pipe.